The present invention relates generally to amorphous memory devices and more specifically to an improved method of setting amorphous memory devices.
Amorphous semiconductor memory devices, for example a tellurium base chalcogenide glass, are well known. These memory devices are generally bi-stable devices including the film of memory semiconductor material capable of being switched from a stable high resistance amorphous state into a stable low resistance crystalline state when a write or set voltage pulse of relatively long duration is applied. Such a set voltage pulse causes current to flow in a small filament. The resulting set or write current generally heats the region of the semiconductor material above its glass transition temperature into its crystallization temperature such that the material crystallizes around and in the region of the conducting filament. A crystallized low resistance filamentous path remains indefinitely even when the voltage and current are removed, until a reset or erase pulse is applied to return the filamentous path to its initial amorphous high resistance state.
A typical applied set voltage pulse V.sub.A is illustrated in FIG. 1 for a write. The resulting voltage across the device is illustrated in FIG. 2. Once the voltage across the device reaches the threshold voltage V.sub.TH, the device switches from its high resistance state to a low resistance state. Upon reaching the low resistance state, the voltage levels off at substantially V.sub.H which is the holding voltage. As illustrated in FIG. 3, the current through the device begins to flow substantially after the device switches and continues until the voltage is removed.
The mechanisms involved in the material are illustrated in FIGS. 4, 5 and 6. As illustrated in FIGS. 4A and 5A, the voltage across the device creates a filamentous path in the center between the electrodes. Once the threshold voltage is exceeded, a complete filamentous path is established and the material switches as illustrated in FIGS. 4B and 5B. Since the device desires to maintain a constant current density, the filamentous path expands radially to accommodate the excess current. The temperature in the filamentous path is too high for the crystallization of the material. A given distance away from the hot filamentous path, the temperature is favorable for crystallization and an annular resistor forms about and separates from the hot filamentous path as illustrated in FIG. 4C. The resistor path starts stealing current from the hot filamentous path as illustrated in FIGS. 5C and 6C. Thus, the hot filamentous path starts shrinking since it does not have sufficient current to maintain the original radius of the hot filamentous path as illustrated in FIGS. 4D, 5D and 6D. Eventually, the hot filamentous path is extinguished and the annular resistor conducts all of the current as illustrated in FIGS. 4E, 5E and 6E. The resulting structure of FIG. 4E includes a primary filament represented by the circles and a secondary filament represented by the x's. The primary filament is the radius of the original hot filament and the second filament is the annulus crystalline area which encompasses the primary filament. It should be noted that FIG. 4 has an enlarged horizontal scale to emphasize the principle of operation.
In an erase cycle, not all of the crystallites are removed especially in the secondary filament. Thus, in subsequent write cycles the secondary filament will switch before the primary filament thus causing an expansion of the radius of the primary and secondary filament as illustrated in FIG. 7A. Eventually after a large number of set/reset cycles, the filament includes the total radius of the device with an amorphous material center as illustrated in FIG. 7B. This is the locked ON state and cannot be erased without totally destroying the material.
The applied voltage pulse of FIG. 1 is typically about 20 volts resulting in a write current of 6 to 15 milliamps. The pulse width is approximately 10 milliseconds with a one millisecond rise time and one millisecond fall time which are generally a function of the circuit parameters.
The problem with overdriving the filament using the voltage pulse of FIG. 1 is discussed in detail in United Kingdom Patent No. 1,372,414 published Oct. 30, 1974 to Marconi Company Limited. The solution provided by the Marconi patent is to apply a voltage pulse for a predetermined period at least sufficient to drive it into its conducting state but insufficient to render the conductive state permanent and also providing a second low level current pulse for maintaining the current flow through the device after it has been driven into the conductive state. These two pulses are illustrated in FIG. 8 wherein the first voltage pulse P1 is greater than the threshold voltage and may be as high as two times the threshold voltage. The second voltage pulse P2 is smaller than threshold voltage and produces a one milliamp holding current. The pulse width of the pulse P1 is 10 microseconds and the pulse width of P2 is 10 to 100 milliseconds. Since the duration of the first current pulse P1 is not sufficient to render the device permanently conductive, it is applied after the second pulse P2 has reached its desired value so as to maintain the filament in the conductive state for a sufficient time to be permanently conductive.
An important feature of the Marconi patent is that the set current is greater than the first critical value I.sub.1 and less than a second critical current value I.sub.2. The current I.sub.1 is the minimum current to produce the crystallization temperature and I.sub.2 is the current level to drive the material into the liquid phase which is conductive but results in the device returning to OFF state if the current is removed suddenly.
As illustrated in FIGS. 9A, 9B and 9C, the Marconi method of writing forms the same primary filament and secondary filament as the conventional write pulse of FIG. 1, but does not spread as quickly as the write pulse of FIG. 1. This results from the amount of time the device is driven with a large current created by the large voltage of P1 compared to the conventional pulse. Since the secondary filament is also present in Marconi, the primary filament area grows until it eventually fails in the totally ON state as illustrated in FIG. 9C.
Thus, there exists a need for a method of writing which assures that the successive switching always occurs in the same geometric channel.